Intake and/or exhaust gas sensors may be operated to provide indications of various intake and exhaust gas constituents. Output from a Universal Exhaust Gas Oxygen (UEGO) sensor, for example, may be used to determine the air-fuel ratio (AFR) of exhaust gas. Indications of intake and exhaust gas oxygen content may be used to adjust various engine operating parameters, such as fueling. As such, the measurement accuracy of an oxygen sensor may be significantly affected by degradation of an element in the oxygen sensor, such as due to sensor element blackening. Oxygen sensor element blackening is a form of degradation which may occur due to operation of the sensor at a voltage which is in the over-potential region of a sensor element when a higher than threshold electric current is generated.
Various approaches have been used to reduce blackening in oxygen sensor elements. In one example approach, shown by Tsukada et al. in US 20120001641, the pumping voltage used in the oxygen pumping cell of the oxygen sensor may be limited to within a threshold voltage. The threshold voltage may correspond to the boundary of the over-potential region of the cell. During a variable voltage operation of the sensor, wherein the sensor operation is shifted between a higher and a lower voltage, each of the lower and the higher operating voltage may not exceed the threshold voltage.
The inventors herein have recognized potential issues with the above mentioned approach. As one example, by limiting the pumping voltage to a threshold limit, accuracy of the oxygen content measurement by the sensor may be reduced. The desired pumping voltage may change based on factors such as gas concentration, and a fixed upper threshold voltage limit may adversely affect sensor operation. Also, the possibility of blackening may vary based on operating temperature of the sensor, and at a higher operating temperature, even if operating within a threshold voltage, blackening of sensor elements may occur. The inventors have also recognized that the operation of the sensor in the variable voltage mode can result in blackening due to the cell overshooting the target pumping voltage during the transition to the higher voltage. The overshooting voltage may place the sensor in the over-potential region (that is, in a region where the higher voltage can cause the electrolyte in the sensor to be partially electrolyzed due to a removal of oxygen from the sensor).
In an alternate approach to control blackening in oxygen sensor elements, a lower ramping rate may be utilized to attain a desired higher voltage in the UEGO sensor cells such that there is a lower possibility of voltage overshoot to the over-potential region. However, the inventors have recognized potential issues with this approach also. As an example, using a lower ramp rate to increase the operating voltage may be time consuming and result in delays in measurements performed by the sensor, thereby adversely affecting sensor operation.
The inventors herein have recognized that the voltage threshold to cross into the over-potential region increases as the operating temperature of the sensor is decreased. Therefore by decreasing the operating temperature of the sensor, the voltage required to blacken the sensor can be raised, enabling the sensor to be operated over a larger range of voltages before sensor blackening is incurred. In one example, the issues described above may be addressed by a method for an engine comprising: during variable voltage operation of an oxygen sensor, reducing occurrence of blackening of an oxygen sensor element by decreasing an operating temperature of the oxygen sensor from a first temperature to a second temperature before transitioning from a lower operating voltage to a higher operating voltage. In this way, by adjusting the UEGO sensor temperature during variable voltage operation of the UEGO sensor, movement of the UEGO cells due into the over-potential region may be reduced, reducing the possibility of sensor blackening.
As one example, during conditions when an exhaust UEGO sensor is operated in a variable voltage mode, such as for exhaust oxygen content estimation, the temperature of the UEGO sensor may be reduced at least prior to raising the UEGO sensor voltage from a lower, nominal voltage to an upper voltage. By lowering the sensor temperature, a boundary of the over-potential region may be shifted towards a higher absolute voltage. The amount of reduction in UEGO temperature may be determined based on parameters such as a current temperature of the sensor, and the difference between the desired higher voltage and the temperature-modified boundary of the over-potential voltage. The reduction in UEGO temperature may be carried out by adjusting the settings of a heater element coupled to the UEGO sensor so that the heater generates less heat. If it is determined that the boundary of the over-potential region may not be shifted to a desired level only by lowering the UEGO temperature (such as due to higher ambient temperatures or due to other temperature constraints), the upper voltage may be limited to a threshold voltage at or lower than the boundary of the over-potential region. Then, a lower ramp rate of the voltage may be used to attain the higher voltage within the threshold range in order to reduce voltage overshoots.
In this way, by first lowering the UEGO temperature and then transitioning from a lower voltage to a higher voltage operation of the UEGO sensor, the boundary of the over-potential region may be shifted to a higher voltage value and during operation at the higher voltage, risk of blackening of UEGO sensor elements may be reduced. By enabling a higher value of voltage during variable voltage UEGO operation, a higher accuracy may be achieved in UEGO sensor measurements. Therefore, the operating voltage range of the UEGO sensor may be increased. The technical effect of shifting the boundary of the over-potential region to a higher voltage is that a faster ramp rate may be used to attain the higher voltage without the risk of voltage overshoots into the over-potential region. In addition, the risk of voltage overshoots into the over-potential region during voltage transitions are also reduced. By using a faster ramp rate, the higher voltage may be attained within a shorter time which may increase measurement accuracy. Overall, by effective reduction in risk of UEGO element blackening, degradation of the oxygen sensor is reduced, and accuracy of oxygen sensor operation is maintained, enabling efficient engine performance.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.